I'm seriously considering a boat for sale. Its got a 500EFI with 220 total hrs. The guy told me he rebuilt it and it has 30hrs on the rebuild. I asked him why he rebuilt it so soon and he said that he burned 2 pistons with old gas (low octane). I didn't think that old gas could do that much damage to a boat. So my question is what does it take to burn two pistons on a 500EFI? Can old gas do this much damage to the 500EFI. Is the 500EFI that fragile of an engine. I believe the year of the engine is a 2000-2001. Any help/advice/comments would be greatly appreciated.

Maybe he meant he detonated two pistons. Easy to do withtoo much timing and bad gas.

I did it in 2 hours on a brand new (rebuilt) 500EFI.

Old gas = lower octane = detonation

Kennyo - I don't know chit about engines - detonation was the word I was looking for.

T500hps - How did you do that in just 2 hours. Are these engines that fragile. I just want a reliable engine and something I can get in and turn and go. Is a 500EFI something I'm going to have to pamper....... I don't have time or money to have it in the shop all the time and I only run about 15-30hrs per summer. How much did it cost to get yours fixed after the pistons detonated?

Being that I will only run it 15-30hrs per summer am I going to be able to just add fuel stabilizer after a long winter or do I need to empty the tank everytime I don't drive it for 2weeks to a month. I never had any problems with my 502 crate carberated and I had really old gas in her (8months)- I just added fuel stabilizer and turned the key. Never gave me any problems. Same with the 6.2 we have in a sea ray.

When you start detonating a motor it does not take long for the pistons to burn holes in them. Good wot run, bad gas, pluged injector(s), or high engine heat & rebuild time.

This can happen to the best of motors.

respectfully disagree. if motor was built properly to start with , then pinging a hole in a piston or two is strictly a function of someone screwing up.

if it had bad gas due to age ( low octane as mentioned above) then its the guys fault that was dumb enough to try to run the boat w/ old gas...

same w/ the timing and injectors or jets or anything else...

nothing is plug and play when you are talking about high hp big v-8s ... its your job to make sure all that stuff is correct before you launch. if you are going to only run the boat a few hours a year then you solve the gas problem by not filling it full of gas and then letting it sit around... you keep absolute min fuel in it and fuel it fresh each time you go out and if its going to sit for 6 months then you drain it and make sure you run some marvel mystery oil thru the injectors and pumps...etc etc etc...

nothing " just happens"

Don't the M.P.I./E.F.I. engines have knock sensors that can retard timing if needed to save engine from detonation?

yes my 1999-502 MPI/EFI has a knock sensors mounted in the block.